| Author |
Message |
David Stutzman
Senior Member
Username: david_stutzman
Post Number: 29
Registered: 07-2002
| | Posted on Wednesday, December 22, 2004 - 10:30 am: |    |
The project spec included a sheet vapor retarder above a T&G structural wood deck over an indoor pool. The contractor decided the vapor retarder wasn't needed. Now vapor is condensing above the wood deck and finding its way back to the interior through the decking side and end joints.
Does anyone have experience with coatings or other treatments that can be applied to the interior side of the wood deck to act as a vapor retarder? The decking has a stain finish. No coatings have been applied, as far as I know.
Removing the shingle roofing and nailable insulation and installing the specified vapor retarder is the right way to go, but the building is occupied and the owner wants to avoid this scenario, if possible. |
Steven Hauk
New member
Username: sh1net
Post Number: 1
Registered: 12-2004
| | Posted on Friday, December 24, 2004 - 01:52 pm: | |
On the decision as to whether or not to install the vapor barrier, I wonder if that really mattered. After all, the vapor is already inside the building...where the pool is. If one were to install a vapor barrier, doesn't that just provide a surface on which the condensation forms? It seems to me like the condensation will simply form on the highest indoor surface, unless the indoor relative humidity is reduced. Or am I missing something? |
Phil Kabza
Senior Member
Username: phil_kabza
Post Number: 81
Registered: 12-2002
| | Posted on Sunday, December 26, 2004 - 10:24 pm: | |
Pools produce huge vapor drive. I've had similar problems with pools located inside pre-engineered metal buildings, which are notoriously difficult to vapor-proof. A ceiling coating with low permeance could reduce or prevent the vapor from permeating the wood decking and condensing when it reached dew point within the roof assembly. But the cracks between the wood deck planks are the primary source of air and vapor movement to the exterior. Don't know that a vapor retarder sheet would have solved everything, as we don't know from your comments about the overall assembly profile or climate. If the R value was high enough to the outside of the intended vapor retarder, you would theoretically have been okay. In reality, gaps and fasteners that seem minor cause big headaches in assemblies like this.
No matter what you do with the ceiling, the pool area had best be run under negative air pressure. It's tough to get the humidity down to manageable via the HVAC system, but at least you can avoid driving moist air up into the roof assembly. |
John Regener, AIA, CCS, CCCA, CSI, SCIP
Senior Member
Username: john_regener
Post Number: 202
Registered: 04-2002
| | Posted on Sunday, December 26, 2004 - 11:26 pm: | |
It seems that an analysis of the problem would help in defining a solution --- if there is a solution.
The water vapor has condensed above the wood deck. This appears to have been the design intent because the design included a vapor retarder above the deck. Because the vapor has condensed, it has hit a vapor retarding material sufficiently cool enough to cause condensation.
The wood decking probably has a high moisture content now. So, if a vapor retarder is installed below the decking, then there will still be moisture that will move up. Think summer time condition. Somehow the wood decking needs to dry. Some provision needs to be made for this moisture to come out.
On the underside, there is a new problem of securing a vapor retarder to the underside of the wood decking to stop the vapor drive. In a cold storage facility, a vapor retarder completely encloses the interior side of the walls and ceilings. A reinforced polyethylene sheeting, with the joints lapped and sealed with appropriate tape could produce an effective barrier. Over this vapor barrier, water-resistant and finishable gypsum board (gypsum soffit board) could be applied and finished with chemical-hardening joint compound and topping. A moisture-resistant coating (epoxy paint with appropriate primer) could be applied. The epoxy paint itself has a low perm rating.
I think the key is to stop the water vapor at the underside of the wood decking. |
David Stutzman
Senior Member
Username: david_stutzman
Post Number: 31
Registered: 07-2002
| | Posted on Monday, December 27, 2004 - 09:09 am: | |
John,
The vapor barrier above the wood deck was in the right place to prevent condensation, had the vapor barrier been installed.
We have not asked the contractor to take the roof apart yet, so we cannot say where exactly the condensation is occurring. I suspect it is occurring within the 3 inch thick nailable insulation. The project is in New Jersey and except for a couple of days, low daily temperatures have been well above the seasonal norm and now where near the design temperature, yet. I am still trying to get information on all the products that were actually installed in the roof assembly.
Thank you for the comment about allowing the wood to dry. This is an excellent point and must be completed before any coating is applied. Your solution for an interior barrier is workable. I have used the same approach for cold warehouses. I doubt the owner will want to conceal the wood because of the "look," but we can explore that possibility.
I did learn that the architect was not aware that the spec required a vapor barrier above the deck. I believe this may put the responsibility on the Contractor to devise a solution, if he does not want to remove the roof and insulation and install it correctly. |
David Stutzman
Senior Member
Username: david_stutzman
Post Number: 32
Registered: 07-2002
| | Posted on Monday, December 27, 2004 - 09:43 am: | |
Steven,
Condensation occurs whenever moist air contacts a surface at or below the dewpoint of the moist air. The dew point is a function of the air temperature (dry bulb),saturation temperature (wet bulb), enthalpy (btu/pound of dry air), and humidity ratio (pounds of moisture per pound of dry air). Dew point temperatures can be calculated when at least two of these components are known. The easiest way to determine dew point temperatures is by a psychrometric chart that plots all these functions on the same graph.
The point within the space makes no difference for condensation because the vapor pressure is evenly distributed within the volume of moist air. The vapor will penetrate the surrounding construction at the rate permitted by the materials involved until it contacts a surface at or below the dew point.
The point of the vapor barrier is to prevent moist air from penetrating an assembly to the point at which the temperature within the assembly will cause condensation. Condensation if not contained and weeped to the exterior, can deteriorate construction assemblies by corrosion, rot, and freeze/thaw cycling. |
Steven Hauk
Junior Member
Username: sh1net
Post Number: 2
Registered: 12-2004
| | Posted on Monday, December 27, 2004 - 10:47 am: | |
David,
I understand dew points and vapor barriers. I also understand the harm moisture can cause in construction.
My question/comment was to suggest that maybe your heated pool is behaving like a steam room, where moisture is so dense in the air it collects on the ceiling and drips down. Of course, it collects on the walls, too, but that's less noticeable. Maybe that never happens. I don't really know.
To suggest that an adequately insulated room in a region experiencing unseasonably warm temperatures is experiencing condensation simply because of the lack of a vapor barrier seems like a diagnosis that could be questioned. By the way...condensation *within* nailable insulation? Have you really seen such an occurrence? I'll stay out of the conversation from here. I'm no expert. |
David Stutzman
Senior Member
Username: david_stutzman
Post Number: 33
Registered: 07-2002
| | Posted on Monday, December 27, 2004 - 11:47 am: | |
Steven,
I'm sorry I misunderstood your question. Please don't let that keep you from participating in the discussion.
I don't believe the room is acting as a steam room. The HVAC system is supposedly designed to control the humidity and maintain the RH at a reasonable level. The condensation did not appear until the temperatures turned colder. The architect reported that the moisture appears to be dripping from the T&G joints. There was no mention of the deck surface being covered with condensation. The architect's reports suggest the condensation is occurring above the wood deck. |
Steven Hauk
Member
Username: sh1net
Post Number: 3
Registered: 12-2004
| | Posted on Monday, December 27, 2004 - 12:29 pm: | |
David,
I don't mean to imply that I was insulted...only that I don't know that my guess at a possible alternate diagnosis is worth exploring. |
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